114 CLIMATIC CYCLES AND TREE-GROWTH 



Turner's meteor-swarm theory has the merit of simplicity, since 

 it merely becomes an extension of the accretion hypothesis (Chamber- 

 lin and Moulton) and offers many choices in periods. Perhaps size 

 and shape of a meteor swarm could be invoked to explain crudely the 

 butterfly diagram, but it is exceedingly difficult to reach with this 

 theory the polarity and rotation of spots. 



Short-period cycles in sunspots — An analysis of monthly sunspot 

 numbers since 1750 gave a number of possible cycles, of which 7.9 

 months and especially 10.5 months were the best. The former of 

 these is the period required by a meteor swarm to pass in a very 

 elliptical orbit out to the orbit of Mars and back to the sun. The 

 latter is the period a swarm would have with aphelion near the inner 

 asteroids. The various periods noted in monthly sunspot numbers 

 were found to be multiples of 35 days, which is very nearly the sidereal 

 time of polar rotation of the sun (Abbot, 1925, p. 100). But to the 

 present time no one has found any satisfactory evidence of planetary 

 influence in the formation of sunspots, and this coincidence may be 

 accidental. If there were a tidal effect from any planet, it would 

 presumably take place twice in the solar rotation. 



Solar rotation — Adams and others have applied the spectroscope 

 to solar rotation at different latitudes and find sidereal periods for 

 average surface rotation as follows: latitude 0°, 24.6 days; 30°, 26.3 

 days; 60°, 31.2 days; 80°, 35.3 days. High levels in the solar atmos- 

 phere rotate faster at all latitudes. 



Radiation — Abbot (1925) has done important work upon radiation, 

 and now has an accurate record of the solar constant from 1918 on. 

 The values passed below normal in 1922 and stayed so during the 

 sunspot minimum of 1923. With the beginning of the new sunspot 

 cycle this constant has come back to normal. All this change seems 

 to be a correlation with the sunspot cycle, with radiation 3 per cent 

 above normal at the maximum activity. However, this is subject to 

 sudden brief decreases, reaching even 10 per cent, when unusually 

 large spot-groups are about one day past the sun's central meridian. 



Ultra-violet radiation — Pettit and Nicholson (1926) have con- 

 structed a recorder of ultra-violet radiation (which has a powerful 

 effect on plant life), using a thin silver film as screen and producing 

 galvanometer deflections by a thermo-couple. The variations follow 

 the sunspot activity with accuracy and at the same time exhibit a far 

 greater sensitiveness to its changes than found in the solar-constant 

 records, reaching perhaps 80 per cent difference between readings at 

 times of maximum and minimum sunspot activity. The instrument 

 promises to be of unusual value. Perhaps in this way will come the 

 solution of a problem formulated years ago on finding the remarkable 

 solar records in trees around the Baltic Sea. 



